172 BELL SYSTEM TECHNICAL JOURNAL 



ears of the telephone user is converted by the transmitter into electrical 

 currents, a part of which actuates the receiver, thus producing noise. 

 The present trend in telephone practise is very strongly toward a 

 reduction of these effects. This will tend to bring into increasing 

 prominence noise caused by induction in the telephone circuits which 

 now in many cases is partially overshadowed by the reproduction of 

 the noises in the room. 



As partly offsetting this tendency steps have been taken to improve 

 the degree of balance to ground of new station apparatus, particularly 

 in the case of party lines. The new station apparatus with the 

 improved transmission characteristics discussed above will be designed 

 for reduced effect of noise currents entering from the line. Also, in 

 extending the selective signaling features to rural areas, higher im- 

 pedance ringers and a newly developed high impedance relay are being 

 used in order to limit susceptiveness to noise from exposures between 

 the rural open wire extensions and rural power distribution circuits. 

 Where central office equipment is being modified to permit of increased 

 range of direct current signaling, or for some other reason, the reduction 

 of susceptiveness is always a consideration. All of the newer repeating 

 coils used for supplying talking battery to subscribers in common 

 battery areas, which comprise the bulk of the local plant, possess a 

 much higher degree of balance than the coils which were standard a 

 few years ago. 



Other Items. — So far the changes which are associated directly with 

 the major trends of development in the telephone plant have been 

 described. The broad outlines of these developments depend on all of 

 the factors affecting telephone service as well as coordination with 

 power circuits. There are other features not directly associated with 

 these main trends which, while introduced into the telephone plant 

 largely because of the advantages to be gained in reducing susceptive- 

 ness to electrical influences, have also afforded other benefits. A few 

 of the more interesting examples of these changes are given below. 



Referring to the toll plant, there may be mentioned the recently 

 adopted general practise of soldering aerial wire sleeve connections in 

 order to insure a permanently high degree of series balance. Hereto- 

 fore reliance had been placed on the contact between the wires and the 

 twisted sleeve. The practise of soldering will be supplemented in the 

 near future by a cold-rolled sleeve method, and it is confidently ex- 

 pected that these practises will result in material noise improvements. 

 They will also probably reduce the maintenance required on open wire 

 toll circuits, particularly where exposures are involved. 



Another item is the abandonment of the use of iron wire and sub- 



